1. Field of the Invention
The present invention relates to the field of electrodes and ion selective membranes for use therewith, particularly those membranes and electrodes specifically responsive to carbonate ions such as are related to the carbon dioxide content of human blood, serum or plasma.
2. Brief Description of the Prior Art
The normal pH of plasma is 7.4 and defines the ratio of HCO.sub.3.sup.- to H.sub.2 CO.sub.3 (by the Henderson-Hasselbalch equation). Disturbances in blood pH are compensated by appropriate responses of the respiratory and renal systems. Hence, more than one analysis is required to determine acid-base status. One such determination is the analysis of the carbon dioxide content of the blood. Carbon dioxide dissolved in blood is in equilibrium between the interior of red blood cells and the plasma and also within the plasma. It is present as dissolved carbon dioxide (CO.sub.2), carbonic acid (H.sub.2 CO.sub.3), bicarbonate (HCO.sub.3.sup.-), carbonate (CO.sub.3.sup.-2) and carbonate bound to free amino groups of proteins (RNHCOO.sup.-. The total concentration is defined as the sum of the concentrations of all forms in which CO.sub.2 is present. In most methods for the determination of total carbon dioxide in blood, serum or plasma, the biological fluid is added to an acid reagent which converts bound CO.sub.2 (HCO.sub.3 --, CO.sub.3.sup.-2 and RNHCOO.sup.-) into free CO.sub.2 (H.sub.2 CO.sub.3 and dissolved CO.sub.2). As a measure of total carbon dioxide, one may employ extraction methods (e.g., dialysis) or equilibration methods, measuring increase in pressure of gas at a fixed volume. Potentiometric determination of the total CO.sub.2 has been performed using carbonate ion-selective electrodes. This requires the fixing of the pH of the sample at a relatively high value by the addition of a buffered solution prior to testing, such as described in Herman and Rechnitz, Anal. Chim. Acta, 76:155-164 (1975).
Wise, U.S. Pat. No. 3,723,281, describes a bicarbonate ion sensitive electrode in which the ion sensing portion of the electrode is an organic solution containing a high molecular weight quaternary ammonium salt dissolved in dual solvent system consisting of a trifluoroacetyl-p-alkyl benzene and an alcohol of low water solubility. Electrodes of this sort which contain liquid-sensing components have shorter life spans and are less stable and selective than others in solid format which have more recently become known in the art.
Kim, et al, U.S. Pat. No. 4,272,328, describes an ion selective electrode multilayer analytical element which includes an ionophore-containing membrane and a buffer zone which comprises a hydrophilic binder and a buffer in an amount sufficient to control the pH of the solution as analyzed between about 7.5 to about 9.5. Under these conditions it is asserted that the element is comparatively less susceptible to the interfering effects of gentisate, salicylate and p-amino salicylate. Nowhere does this describe the avoidance of interference from species such as heparin or from large molecular weight molecules endogenous to serum such as free fatty acids, triglycerides or lipids. Apart from the separate buffer overcoat, it nowhere describes the use of particular molecules in the ionophore membrane effective to exclude ions other than carbonate ions to enhance selectivity. The only mention which is made of hydrophobic binders is with reference to the polyvinyl chloride used as the polymeric inert support material. The use of quaternary ammonium salts is generally discussed and examples are given only of quaternary salts in which the substitutions include lower alkyl constituents.
Avoidance of the effects of interfering species has been sought in various potentiometric determinations. For example, a chloride ion-selective electrode has been described which uses a solid polymer membrane. The membrane is essentially a plasticized polystyrene film with a proper quaternary ammonium ion and also includes an appropriate sulfonic acid group in order to repel anions other than chloride. See, Oka, et al, Anal. Chem. 53:588-593 (1981).
Meyerhoff, et al, Anal. Chim. Acta, 141:57-64 (1982) describe a carbonate selective polymeric membrane electrode in which the quaternary ammonium ion is Aliquat 336, the ionophore is trifluoroacytl-p-butylbenzene, and the plasticizer is either di-(2-ethylhexyl) sebacate or di-octyl phthalate and the polymeric inert support material is poly (vinyl chloride). The data reported in this publication provide a relatively slow response time, particularly as shown in FIG. 4 thereof, and relatively significant effects of interference from salicylate.
A recent publication, Simon, et al, Anal. Chem., 54:423-429 (1982) describes a bicarbonate electrode which uses ethanolamines as ion-exchangers, but interference from the biologically common chloride anion is present. The device has excessively long response times (5-15 minutes).
In summary, potentiometric sensors have been designed to avoid interference from like-charged ions by incorporating ionized species into the membrane body. These serve to physically repel the interfering ions from the membrane body and thereby minimize their effect on the development of the electrode potential. Layered structures may act as diffusion barriers to interfering species (e.g., via adsorption) but response time is necessarily slow.